The structure of hyperalkaline aqueous solutions containing high concentrations of gallium – a solution X-ray diffraction and computational study

Abstract

Highly concentrated alkaline NaOH–Ga(OH)₃ solutions with 1.18 M ≤ [Ga(III)]_T ≤ 2.32 M and 2.4 M ≤ [NaOH]_T ≤ 4.9 M (where the subscript T denotes total or analytical concentrations) have been prepared and investigated by solution X-ray diffraction and also by ab initio quantum chemical calculations. The data obtained are consistent with the presence of only one predominant Ga(III)-bearing species in these solutions, which is the tetrahedral hydroxo complex Ga(OH)₄⁻. This finding is in stark contrast to that found for Al(III)-containing solutions of similar concentrations, in which, besides the monomeric complex, an oxo-bridged dimer was also found to form. From the solution X-ray diffraction measurements, the formation of the dimeric (OH)₃Ga–O–Ga(OH)₃²⁻ could not unambiguously be shown, however, from the comparison of experimental IR, Raman and ⁷¹Ga NMR spectra with calculated ones, its formation can be safely excluded. Moreover, higher mononuclear stepwise hydroxo complexes, like Ga(OH)₆³⁻, which have been claimed to exist by others in the literature, were not possible to experimentally detect in these solutions with any of the spectroscopic techniques used.